Zinc nitride compound and method for producing same

ABSTRACT

The present invention provides a zinc nitride compound suitable for electronic devices such as high-speed transistors, high-efficiency visible light-emitting devices, high-efficiency solar cells, and high-sensitivity visible light sensors. The zinc nitride compound is represented, for example, by the chemical formula CaZn 2 N 2  or the chemical formula X 1   2 ZnN 2  wherein X 1  is Be or Mg. The zinc nitride compound is preferably synthesized at a high pressure of 1 GPa or more.

TECHNICAL FIELD

The present invention relates to a zinc nitride compound and a methodfor producing the same.

BACKGROUND ART

GaN is widely used in LED light sources. However, GaN has a wide bandgap and therefore cannot emit light in the visible range by itself. InNhas too narrow a band gap, due to which it cannot emit light in thevisible range. Ga is a high-cost element, and In is a scarce element.Various zinc nitride compounds have been proposed; however, for example,ZnSnN₂ is difficult to produce in the form of a p-type semiconductor,and Ca₂ZnN₂ is an indirect band gap semiconductor. The problem withthese zinc nitride compounds is that, for example, they are unsuitablefor use in light-emitting devices or high-efficiency solar cells (NonPatent Literature 1).

CITATION LIST Non Patent Literature

Non Patent Literature 1: J. Solid State Chem. 88, 528-533 (1990)

SUMMARY OF INVENTION Technical Problem

It is an object of the present invention to solve the above problem andprovide a zinc nitride compound suitable for electronic devices such ashigh-speed transistors, high-efficiency visible light-emitting devices,high-efficiency solar cells, and high-sensitivity visible light sensors.

Solution to Problem

The present disclosure provides the following inventions to solve theabove problem.

(1) A zinc nitride compound represented by the chemical formula CaZn₂N₂.

(2) A zinc nitride compound represented by the chemical formula X¹ ₂ZnN₂wherein X¹ is Be or Mg.

(3) A zinc nitride compound represented by the chemical formula Zn₃LaN₃.

(4) A zinc nitride compound represented by the chemical formula ZnTiN₂.

(5) A zinc nitride compound represented by the chemical formula ZnX²N₂wherein X² is Zr or Hf.

(6) A zinc nitride compound represented by the chemical formula Zn₂X³N₃wherein X³ is V, Nb, or Ta.

(7) A zinc nitride compound represented by the chemical formula Zn₃WN₄.

(8) The zinc nitride compound according to any one of (1) to (7), beinga compound semiconductor.

(9) The zinc nitride compound according to (1) or (3), being a directband gap compound semiconductor.

(10) A compound semiconductor represented by the chemical formula CaM¹_(2x)Zn_(2(1-x))N₂ wherein M¹ is Mg or Cd and 0≤x≤1 or M²_(x)Ca_(1-x)Zn₂N₂ wherein M² is Sr or Ba and 0≤x≤1, the compoundsemiconductor having a band gap of 0.4 eV to 3.2 eV.

(11) An electronic device including an active layer including thecompound semiconductor according to any one of (8) to (10).

(12) The electronic device according to (11), wherein the electronicdevice emits light in the visible range under current injection.

(13) The electronic device according to (11), wherein the electronicdevice generates a photovoltage or a photocurrent by absorbing visiblelight.

(14) A method for producing the zinc nitride compound according to anyone of (1) to (7), including synthesizing the zinc nitride compound at ahigh pressure of 1 GPa or more.

Advantageous Effects of Invention

The zinc nitride compound of the present invention can be provided as azinc nitride compound suitable for electronic devices such as high-speedtransistors, high-efficiency visible light-emitting devices,high-efficiency solar cells, and high-sensitivity visible light sensors.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 shows the basic electronic properties of zinc nitride compoundsaccording to the present invention.

FIG. 2a shows the crystal structure of the zinc nitride compoundrepresented by the chemical formula CaZn₂N₂, FIG. 2b shows thecalculated phase diagram of the Ca—Zn—N system, FIG. 2c shows the bandstructure (conduction band and valence band) of the zinc nitridecompound represented by CaZn₂N₂, and FIG. 2 d shows the phase diagram ofthe Ca—Zn—N system in the chemical potential space.

FIG. 3 shows the crystal structures of typical zinc nitride compounds ofthe present invention other than the zinc nitride compound A)represented by the chemical formula CaZn₂N₂.

FIG. 4 shows the calculated phase diagrams of the systems such as theMg—Zn—N system for the typical zinc nitride compounds of the presentinvention other than the zinc nitride compound A) represented by thechemical formula CaZn₂N₂.

FIG. 5 shows the band structures (conduction bands and valence bands) ofthe typical zinc nitride compounds of the present invention other thanthe zinc nitride compound A) represented by the chemical formulaCaZn₂N₂.

FIG. 6 shows X-ray diffraction patterns of synthesis products obtainedin Example 1 and Comparative Example 1.

FIG. 7 shows absorption spectra of the synthesis products obtained inExample 1 and Comparative Example 1.

FIG. 8 shows X-ray diffraction patterns of a synthesis product obtainedin Example 2.

FIG. 9 shows photoluminescence spectra of the synthesis product obtainedin Example 2.

FIG. 10 shows X-ray diffraction patterns of a purified powder productobtained in Example 2.

DESCRIPTION OF EMBODIMENTS

The zinc nitride compound of the present invention can be represented byany one of the following chemical formulae.

A) A zinc nitride compound represented by the chemical formula CaZn₂N₂.

B) A zinc nitride compound represented by the chemical formula X¹ ₂ZnN₂wherein X¹ is Be or Mg.

C) A zinc nitride compound represented by the chemical formula Zn₃LaN₃.

D) A zinc nitride compound represented by the chemical formula ZnTiN₂.

E) A zinc nitride compound represented by the chemical formula ZnX²N₂wherein X² is Zr or Hf.

F) A zinc nitride compound represented by the chemical formula Zn₂X³N₃wherein X³ is V, Nb, or Ta.

G) A zinc nitride compound represented by the chemical formula Zn₃WN₄.

The zinc nitride compounds A) to G) according to the present inventionare novel compounds which are not included in Inorganic CrystalStructure Database (ICSD). The basic electronic properties of the zincnitride compounds A) to G) according to the present invention are shownin FIG. 1. In FIG. 1, a shows the band gaps (● represents a direct bandgap, while ∘ represents an indirect band gap), and b shows the effectivemasses of holes and electrons. The zinc nitride compounds according tothe present invention are compound semiconductors. In particular, thezinc nitride compound A) or C) according to the present invention whichis represented by the chemical formula CaZn₂N₂ or Zn₃LaN₃ is a directband gap compound semiconductor and suitable for use, for example, inlight-emitting devices and thin-film solar cells.

The space groups to which the zinc nitride compounds according to thepresent invention belong are as follows.

The zinc nitride compound A) belongs to the space group P-3m1.

The zinc nitride compound B) belongs to the space group I4/mmm.

The zinc nitride compound C) belongs to the space group P6₃/m.

The zinc nitride compound D) belongs to the space group Pna2₁.

The zinc nitride compound E) belongs to the space group P3m1.

The zinc nitride compound F) belongs to the space group Cmc2₁.

The zinc nitride compound G) belongs to the space group Pmn2₁.

Hereinafter, the zinc nitride compound A) represented by the chemicalformula CaZn₂N₂, which is suitable as a direct band gap compoundsemiconductor, will be described as a representative of the zinc nitridecompounds of the present invention. Understanding of the other zincnitride compounds denoted by B) to G) can also be gained from the entirecontents of the detailed description.

In FIG. 2, a shows the crystal structure of the zinc nitride compound A)represented by the chemical formula CaZn₂N₂, b shows the calculatedphase diagram of the Ca—Zn—N system, c shows the band structure(conduction band and valence band) of the zinc nitride compoundrepresented by CaZn₂N₂, and d shows the phase diagram of the Ca—Zn—Nsystem in the chemical potential space. As seen from d, CaZn₂N₂ isstable at a high nitrogen chemical potential, namely at a high nitrogenpartial pressure.

The zinc nitride compound A) represented by the chemical formula CaZn₂N₂has a band gap of 1.9 eV. The band gap can be controlled byincorporation of Mg, Sr, Ba, or Cd, which results in a compoundsemiconductor represented by the chemical formula CaM¹_(2x)Zn_(2(1-x))N₂ wherein M¹ is Mg or Cd and 0≤x≤1 or M²_(x)Ca_(1-x)Zn₂N₂ wherein M² is Sr or Ba and 0≤x≤1 and having a band gapof 0.4 eV to 3.2 eV. CaZn₂N₂ can be an n-type semiconductor even whenundoped, since nitrogen holes serving as shallow donors are likely to begenerated in CaZn₂N₂. However, it is preferable to form an n-typesemiconductor by doping into CaZn₂N₂. Furthermore, for example, a highernitrogen partial pressure reduces carrier compensation by nitrogenholes, thus allowing the formation of a p -type semiconductor by dopinginto CaZn₂N₂.

The zinc nitride compound A) represented by the chemical formula CaZn₂N₂is preferably synthesized at a high pressure of 1 GPa or more. In thiscase, the starting compounds, preferably Ca₃N₂ and 2Zn₃N₂, areintroduced into a high-pressure synthesis apparatus and reactedtypically at 800 to 1500° C. and 1 to 10 GPa for about 30 minutes to 5hours.

The resulting high-pressure synthesis product can be purified byremoving zinc from the product. It is preferable to put a powder of theresulting high-pressure synthesis product and I₂, for example, into aglass vessel and place the glass vessel in an atmosphere of an inert gassuch as argon or nitrogen at about 15 to 30° C., preferably at roomtemperature, for about 5 to 10 minutes, thereby converting zinc to zinciodide (ZnI₂). The zinc iodide produced is then dissolved in a solventsuch as dimethyl ether, and the solution is removed. The purpose ofusing an inert atmosphere is to inhibit oxidation of Zn²⁺ and I⁻.

The zinc nitride compound of the present invention can be obtained notonly by the above high-pressure synthesis but also by depositing thecompound as a thin film on a substrate using a physical vapor depositionprocess such as sputtering, pulsed laser deposition, or vacuumdeposition or a chemical vapor deposition process such as organometallicchemical vapor deposition. The substrate can be selected as appropriatedepending on the intended purpose and, for example, an oxide substratemay be used.

CaZn₂N₂ is particularly useful in that it is composed of elementsabundant on the earth, it has a direct band gap, and its charge carriershave a small effective mass (the effective mass of electrons is 0.17 m₀,and the effective mass of holes is 0.91 m₀). The direct band gap of 1.9eV of CaZn₂N₂ corresponds to the red region of visible light, and thusCaZn₂N₂ can be expected to show a high theoretical conversion efficiencywhen used as a light absorbing layer of a solar cell. An electronicdevice having an active layer made of CaZn₂N₂ is useful as an electronicdevice (light-emitting device) that emits light in the visible rangeunder current injection or as an electronic device (solar cell or lightsensor) that generates a photovoltage or a photocurrent by absorbingvisible light.

FIGS. 3a to 3e show the crystal structures of the typical zinc nitridecompounds of the present invention other than the zinc nitride compoundA) represented by the chemical formula CaZn₂N₂ just as FIG. 2a shows thecrystal structure of the zinc nitride compound A).

FIGS. 4a to 4g show the calculated phase diagrams of the systems such asthe Mg—Zn—N system for the typical zinc nitride compounds of the presentinvention other than the zinc nitride compound A) represented by thechemical formula CaZn₂N₂ just as FIG. 2b shows the calculated phasediagram for the zinc nitride compound A).

FIGS. 5a to 5g show the band structures (conduction bands and valencebands) of the typical zinc nitride compounds of the present inventionother than the zinc nitride compound A) represented by the chemicalformula CaZn₂N₂ just as FIG. 2c shows the band structure of the zincnitride compound A).

INDUSTRIAL APPLICABILITY

The present invention can provide a zinc nitride compound suitable forelectronic devices such as high-speed transistors, high-efficiencyvisible light-emitting devices, high-efficiency solar cells, andhigh-sensitivity visible light sensors.

EXAMPLES

Hereinafter, the present invention will be described in more detail withExamples.

Example 1

Synthesis of Zinc Nitride Compound Represented by CaZn₂N₂

Starting compounds, Ca₃N₂ and Zn₃N₂ mixed in a molar ratio Ca₃N₂:Zn₃N₂of 1:2, were introduced into a high-pressure synthesis apparatus, whichwas maintained at 2.5 GPa and 1100° C. for 1 hour. The high-pressuresynthesis apparatus used is a belt-type high-pressure synthesisapparatus which has a high-pressure cell as a sample holder, whosepressure control range is from 2 to 5.5 GPa, and whose temperaturecontrol range is from room temperature to 1600° C.

X-ray diffraction patterns of the resulting high-pressure synthesisproduct are shown in FIG. 6a . About 69 wt % of the product consisted ofCaZn₂N₂, and the rest consisted of Zn. As for the lattice parameters ofCaZn₂N₂, the lattice parameters a and c were respectively 3.463150(44) Åand 6.01055(11) Å which differ by 0.3% from the theoretical latticeparameters a and c of 3.454 Å and 5.990 Å.

Absorption spectra of CaZn₂N₂ as obtained through diffuse reflectancespectroscopy and calculation using the Kubelka-Munk equation are shownin FIGS. 7a to 7c , together with absorption spectra of Ca₂ZnN₂ ofComparative Example 1. It is seen that the absorption of CaZn₂N₂ sharplyrises. CaZn₂N₂ had a direct band gap of 1.9 eV (calculated value=1.83eV).

Comparative Example 1

Synthesis of Zinc Nitride Compound Represented by Ca₂ZnN₂

Starting compounds, Ca₃N₂ and Zn₃N₂ mixed in a molar ratio Ca₃N₂:Zn₃N₂of 2:1, were put into an Ar-filled SUS tube, which was placed in anelectric furnace at ordinary pressure and 680° C. for 40 hours. Thesample holder was a reaction tube constituted by SUS gas tubing andSwagelok fittings.

X-ray diffraction patterns of the resulting reaction product are shownin FIG. 6b . The resulting product was Ca₂ZnN₂. As for the latticeparameters of Ca₂ZnN₂, the lattice parameter a was 3.583646(65) Å whichdiffers by 0.2% from the theoretical lattice parameter a of 3.575 Å, andthe lattice parameter c was 12.663346(26) Å which differs by 0.4% fromthe theoretical lattice parameter c of 12.607 Å.

Absorption spectra of Ca₂ZnN₂ as obtained through diffuse reflectancespectroscopy and calculation using the Kubelka-Munk equation are shownin FIGS. 7a to 7c . Ca₂ZnN₂ had an indirect band gap of 1.6 eV(calculated value=1.65 eV) and a direct band gap of 1.9 eV (calculatedvalue=1.92 eV).

When synthesis was attempted in the same manner as in ComparativeExample 1 except for mixing the starting materials in the ratio used inExample 1, CaZn₂N₂ of the present invention was not obtained.

Example 2

Synthesis of Zinc Nitride Compound Represented by CaZn2N2

Starting compounds, Ca₃N₂ and Zn₃N₂ mixed in a molar ratio Ca₃N₂:Zn₃N₂of 1:2, were introduced into a high-pressure cell and subjected tohigh-pressure synthesis in which a pressure of 5.0 GPa was applied at1200° C. for 1 hour. The high-pressure synthesis apparatus used is abelt-type high-pressure synthesis apparatus which has a high-pressurecell as a sample holder, whose pressure control range is from 2 to 5.5GPa, and whose temperature control range is from room temperature to1600° C.

X-ray diffraction patterns of the resulting high-pressure synthesisproduct are shown in FIG. 8. About 80 wt % of the product consisted ofCaZn₂N₂, and the rest consisted of Zn etc. As for the lattice parametersof CaZn₂N₂, the lattice parameters a and c were respectively 3.46380(11)Å and 6.00969(30) Å which differ by 0.3% from the theoretical latticeparameters a and c of 3.454 Å and 5.990 Å.

Light emission, in particular photoluminescence, from CaZn₂N₂ obtainedas the high-pressure synthesis product was examined through photonexcitation induced using a third-harmonic Nd:YAG pulsed laser(wavelength: 355 nm, energy density: up to 7 mJ/cm²). Redphotoluminescence was clearly observed by visual inspection at 10 K. Theresults are shown in FIG. 9. In FIG. 9, a shows photoluminescencespectra obtained at 10 K, 100 K, 200 K, and 300 K, and b shows thetemperature dependence of the spectral peak position.

(Purification of CaZn₂N₂)

A powder of the obtained high-pressure synthesis product (includingCaZn₂N₂ and Zn etc.) and 12 were put into a glass vessel, which wasplaced in an argon atmosphere at room temperature for about 5 minutes toconvert zinc into zinc iodide. The zinc iodide produced was thendissolved in dimethyl ether, and the solution was removed. CaZn₂N₂accounted for about 87.3 wt % of the resulting powder, and zinc etc.accounted for about 12.7 wt % of the powder. X-ray diffraction patternsof the purified powder product are shown in FIG. 10. A pellet for use asa pulsed laser deposition target was able to be formed from about 1 g ofthe purified powder product using a cold isotropic press (CIP) machine.

1. A zinc nitride compound represented by the chemical formula CaZn₂N₂.2. (canceled)
 3. A zinc nitride compound represented by the chemicalformula Zn₃LaN₃. 4-7. (canceled)
 8. The zinc nitride compound accordingto claim 1, being a compound semiconductor.
 9. (canceled)
 10. A compoundsemiconductor represented by the chemical formula CaM¹_(2x)Zn_(2(1-x))N₂ wherein M¹ is Mg or Cd and 0≤x≤1 or M²_(x)Ca_(1-x)Zn₂N₂ wherein M² is Sr or Ba and 0≤x≤1, the compoundsemiconductor having a band gap of 0.4 eV to 3.2 eV.
 11. An electronicdevice comprising an active layer comprising the compound semiconductoraccording to claim
 8. 12. The electronic device according to claim 11,wherein the electronic device emits light in the visible range undercurrent injection.
 13. The electronic device according to claim 11,wherein the electronic device generates a photovoltage or a photocurrentby absorbing visible light.
 14. (canceled)
 15. The zinc nitride compoundaccording to claim 3, being a compound semiconductor.